As drilling operations are conducted in deeper and deeper water, such as 300 feet and more, it becomes impractical and uneconomical to drill from platforms supported by the underwater ground. These drilling operations are better conducted by floating ship-shaped vessels and semi-submersibles. Mooring systems or station-keeping systems for such vessels are used to maintain the vessel on location. It sometimes becomes necessary or desirable to move off location quickly, such as in bad weather, and this has always presented problems for moored vessels since the usual procedure has been to release and drop all anchor chains, or cut a chain link while under tension by torch. To reposition the vessel on location, it then becomes necessary to grapple for the released anchors and anchor chains. High day rates for labor and capital investment dictate the adoption of a design for moving off location quickly and then repositioning the vessel on location quickly and simply thus reducing the time required for such purposes so that drilling can be commenced again as soon as possible.
It would be highly advantageous to provide a mooring system in which the anchor chains can be released from the anchors with only a predetermined minimum of anchor chain left at the bottom, and when re-positioning it would not be necessary to grapple for the submerged portion of anchor chains left on bottom thereby permitting the vessel to be re-positioned on location quickly and by a relatively simple procedure.